Several models of commercial switching amplifiers are known which include class-D amplifiers. Most utilise a system including a first order servo-loop amplifier whose output is connected to a modulation input of a pulse width modulator. An output of the pulse width modulator is connected to an input of an output switching stage, which normally includes switch driver circuits. A negative feedback path connects an output of the output switching stage to an input of the servo-loop amplifier and an amplifier input is also connected to an input of the servo-loop amplifier. This system may be viewed conceptually as the output of the servo-loop amplifier, being an integral of an error signal, the error signal being proportional to the difference between the scaled output of the output switching stage and amplifier input signals. This integrated error signal is that which is fed to the said modulation input.
The pulse width modulator in some systems includes a triangular-wave oscillator which acts as a carrier reference signal, which is applied to an input of a comparator. In some systems which are less common, the carrier reference is a sawtooth waveform rather than a triangular-wave. An output of the servo-loop amplifier is also applied to an input of the comparator. The comparator and the triangular wave oscillator act as the said pulse width modulator, wherein an output of the comparator acts as the output of the pulse width modulator. The servo-loop amplifier most often has a forward transfer function which is an input current to output voltage integrator. For clarity, this system will herein be called the triangular-wave carrier input system.
Another way of implementing the pulse width modulator, is to employ the servo-loop amplifier to produce the said integrated error output added to the triangular-wave carrier reference signal, this mixed sum being applied to an input of the said comparator. This is achieved by adding at the servo-loop integrator's input:                the amplifier input signal, and        the signal at the output of the output switching stage as a negative feedback path, plus        a carrier reference square-wave signal.        
Thus in this system, at the output of the integrator servo-loop amplifier, appears:                a the integrated error signal as described above, plus        an integrated square-wave carrier reference which is thus a triangular-wave carrier reference signal.        
For clarity, this system will herein be called a square-wave carrier input system.
Whilst in a square-wave carrier input system the pulse-width modulator action is intrinsic to the whole circuit, because for example the carrier reference triangular-wave signal requires the closed loop negative feedback for D.C. stability, the pulse width modulator may be viewed in a sense as consisting as the carrier reference square-wave, plus the servo-loop integrating amplifier, plus the comparator. These components are associated with a pulse width modulator forward gain. Reference is made below to this gain. The gain of a pulse width modulator is defined as the ratio between the (change in) duty cycle at the output of the pulse width modulator divided by the (change in) mean output signal at the output of the servo-loop (integrated error) amplifier. For the square-wave carrier input, this is inversely proportional to the amplitude of the carrier reference square-wave fed to the servo-loop amplifier input.
Herein for both systems the servo-loop amplifier, plus carrier references, plus pulse width modulator including comparator shall be referred to as a “servo-loop amplifier plus pulse width modulator.”
The transfer function between the input and output is effectively the same for both these systems. Both these systems employ negative feedback to reduce distortion, that is, improve accuracy. However, these systems are known intrinsically to produce distortion. That is, the systems produce distortion even for perfect electronic components, or in other words, mathematically for idealised components.
In addition, electronic imperfections which are significant, for example in practical power output switching stages, produce further errors.
Details of a system utilising these basic functions is given in Motorola application note AN1042.
A simpler class-D amplifier with no negative feedback or servo-loop amplifier and direct input signal modulation of the pulse width modulator is utilised by a Zetex integrated circuit ZXCD1000. Assuming all components are ideal in such a system concept, this idealised system is known to produce no distortion in contrast to the servo-loop system described above. However, this direct modulation system in practice is known to have several problems compared to the servo-loop approach, namely:
The output noise is typically higher owing to no feedback.
The distortion resulting from practical electronic components is higher at low frequencies where the negative feedback of the servo-loop system is of assistance.
The output signal of the direct modulation system is proportional to the output stage supply ralls and is thus modulated by variations in these rails. Owing to negative feedback, this effect is reduced in the servo-loop system, particularly at lower frequencies which has advantage of more negative feedback.
Class-D amplifiers have been developed by Bang and Olufsen which this company calls its “ICEpower” products. The principles of this system are described in numerous Audio Engineering Society publications and U.S. Pat. No. 6,297,692. This discloses an analogue switching amplifier, in which the overall amplifier dominant pole is set by elements both in the forward servo-loop amplifier paths and also in the negative feedback paths.
My trials with Bang and Olufsen ICEpower models 250A, 500A, 250ASP and 500ASP, yielded the following performance results: the distortion, at 20 kHz at higher powers but below clipping, into 4 ohms appeared to be close to 1% (100 kHz measurement bandwidth). This is roughly 2 orders of magnitude worse than typical well-designed traditional analogue amplifiers. From my general knowledge of this field, I suspect that the ICEpower units tested perform relatively well compared to other commercially available class-D amplifier products.
While I have referred to a specific class-D amplifier that Is commercially available I am aware that units do vary and as such results should of themselves not be necessarily taken as confirmation, but they do suggest that there is some difficulty with such amplifiers.
An object of this invention is therefore to provide an amplifier improvement that assists in reducing distortion or at least provides the public with a useful alternative.